1. Field of the Invention
This invention relates to apparatus for controlling the horizontal scanning rate in beam index color cathode-ray tubes.
2. Description of the Prior Art
Beam index color television receivers are well known in the prior art. They usually include a cathode-ray, or picture, tube having an electron gun which emits a single electron beam and a phosphor screen having a repeating pattern of red, green and blue primary color phosphor stripes extending vertically upon the screen. The beam index picture tube also has a plurality of vertical index phosphor stripes spaced across its phosphor screen in a known relationship to the spacing of the color phosphor strips. When the electron beam horizontally scans the screen, a photodetector generates an index signal in response to the light emitted each time an index stripe is struck by the electron beam. This index signal is used to achieve color registration by controlling the color switching apparatus which determines which of the three primary color signals modulates the intensity of the electron beam, so that the intensity of the electron beam is modulated by the primary color signal whose corresponding phosphor is being scanned by the beam at that moment.
In beam index color television receivers it is important to prevent deviations in the horizontal scanning rate, because such deviations not only distort the shape of images projected upon the picture screen, as they do in other television receivers, but they also cause color misregistration. This misregistration results from the fact that the color switching apparatus which controls when each of the primary color signals modulates the electron beam does not respond instantaneously to the incidence of the electron beam upon the index stripes, but rather responds to the index signal with a delay that varies as a result of changes in the horizontal scanning rate of the beam and thus is difficult to compensate for.
This delay results from the fact that it is common for index signal processing circuitry, for example, comprised of a bandpass filter and a phase-locked loop (PLL) circuit, to be inserted between the photodetector which detects the index signal and the color switching apparatus. The bandpass filter removes unwanted noise from the index signal in preparation for the application of that signal to the input of the PLL circuit. The PLL circuit provides an input to the color switching apparatus which is more uniform in amplitude and frequency than the index signal. In addition, by insertion of a dividing circuit in the feedback loop of the PLL circuit, the latter can be made to produce an output frequency which is a predetermined multiple of the frequency of the index signal. The last feature is important since in most beam index picture tubes the number of color phosphor stripes is not equal to, but instead is an integral multiple of the number of index stripes.
Unfortunately, the delay inherently associated with the above described index signal processing circuitry, particularly with the PLL circuit, varies as a function of the frequency of the index signal, which in turn varies in proportion to the horizontal scanning rate. For this reason, deviations in the horizontal scanning rate adversely affect color registration.
Deviations in the horizontal scanning rate are also disadvantageous in beam index color television receivers because they make it more difficult for the PLL circuit to correctly follow and lock onto the instantaneous frequency of the index signal, as is necessary for proper color registration. In order to cause the PLL circuit to properly follow the frequency of an index signal when the horizontal scanning rate deviates, it is necessary to increase the minimal signal strength of the index signal. This requires that the minimal intensity of the electron beam be increased, which, in turn, has the undesirable result of increasing the luminance of the darkest areas that can be projected on the picture tube, and, thus, of decreasing the contrast of the produced image.
For the above reasons, it is desirable to limit the maximum fluctuations in the horizontal scanning rate of beam index color television receivers to less than several tenths of one percent. In the prior art, various horizontal scanning rate correction methods have been used, but unfortunately none of them has been able to limit such fluctuation to the desired level of less than several tenths of one percent.